JP2007-314780A (Patent Document 1) discloses an antioxidant agent for hot extrusion working, and WO2007/122972 (Patent Document 2) discloses an antioxidant agent for hot plastic working. The antioxidant agents disclosed in these Patent Documents contain a plurality of glass frits having different softening points, and are applied to the surface of a starting material to be subjected to hot plastic working. A metallic starting material to which the antioxidant agent has been applied is heated at a temperature of 800 to 1300° C. in a heating furnace or the like. The antioxidant agents disclosed in Patent Documents 1 and 2 prevent oxides (hereinafter, referred to as scale) from being formed on the surface of the heated metallic starting material.